docs: def get_existing_thumbnail(self, thumbnail_options, high_resolution=False):
          """
          Return a ``ThumbnailFile`` containing an existing thumbnail for a set
          of thumbnail options, or ``None`` if not found.
          """
          thumbnail_options = self.get_options(thumbnail_options)
          names = [
              self.get_thumbnail_name(
                  thumbnail_options, transparent=False,
                  high_resolution=high_resolution)]
          transparent_name = self.get_thumbnail_name(
              thumbnail_options, transparent=True,
              high_resolution=high_resolution)
          if transparent_name not in names:
              names.append(transparent_name)
  
          for filename in names:
              exists = self.thumbnail_exists(filename)
              if exists:
                  thumbnail_file = ThumbnailFile(
                      name=filename, storage=self.thumbnail_storage,
                      thumbnail_options=thumbnail_options)
                  if settings.THUMBNAIL_CACHE_DIMENSIONS:
                      # If this wasn't local storage, exists will be a thumbnail
                      # instance so we can store the image dimensions now to save
                      # a future potential query.
                      thumbnail_file.set_image_dimensions(exists)
                  return thumbnail_file